A lithium ion secondary battery is lighter in weight and has higher capacity than a nickel-cadmium battery, a nickel hydride battery, and the like. Thus, lithium ion secondary batteries are widely used as a power supply for portable electronic devices. The lithium ion secondary battery is also a strong candidate for a power supply mounted on hybrid automobiles or electric automobiles. With the decrease in size and increase in functionality of portable electronic devices in recent years, the lithium ion secondary battery used for the power supply is expected to have higher capacity.
Lithium ion secondary batteries can be made into various forms and a prismatic lithium ion secondary battery, a pillared lithium ion secondary battery and a pouch-type lithium ion secondary battery can be listed as the representative ones.
Among these batteries, the pouch-type lithium ion secondary battery uses a pouch-type case made by sheets, thus, it is light and can be manufactured into various forms of lithium ion secondary batteries. And there is also a strong point in the simple manufacturing process. On the other hand, in the pouch-type lithium ion secondary battery uses a pouch-type case, thus, there is a problem that it is weak to the swelling caused by a flaw or an increase in the inner pressure. The increase in the inner pressure of the lithium ion secondary battery is considered to be caused mainly by the evaporation of the electrolyte solution or the decomposition gas of electrolyte solution.
As the electrolyte solution of the lithium ion secondary battery, it is usually made by mixing a ring carbonate such as ethylene carbonate, a chain carbonate such as diethyl carbonate and adjusting to the desired relative permittivity or viscosity. As the chain carbonate has a lower boiling point, if there is a flaw such as a pinhole in the pouch-type case caused by any reasons in the manufacturing process of the pouch-type lithium ion secondary battery, a part of the electrolyte solution will become vapor and be evaporated and it will result in a bad smell and bad influences such as that the expected discharged capacity cannot be obtained. In addition, when the battery is over-heated or placed under a high temperature, evaporation or a decomposition reaction will happen to the electrolyte solution and the inner pressure will be raised so that swelling will be terrible. Sometimes, the vapor of the electrolyte solution or the decomposition gas of the electrolyte solution will leak from the pouch-type case and the discharged capacity will decrease similarly. Further, problems will rise not only in the safety or the property of the battery, but also may influence the cellular phone or the notebook computer or the like using the battery greatly.
Usually, when the manufactured batteries are to be sold, detection will be provided and the batteries in which leaked gas of electrolyte solution or the like is produced will be picked out. Accordingly, various detection methods for leaked gas of the battery are proposed up to the present.
For example, in Patent document 1, a detection method is proposed for detecting the leaked detection gas from the hermetic battery using a gas sensor by manufacturing a hermetic battery in a hermetically sealed container with the detected gas atmosphere such as helium or argon or the like and then removing the detected gas in the hermetically sealed container followed by decompressing.
However, in the detection method of Patent document 1, a hermetically sealed container is required in the manufacturing process, thus, not only the instrument will be in a large scale, but also a detected gas supplying, pressure reducing devices and processes such as sensing for detected gas using a sensor are required. Therefore, there is a problem that the detection cannot be carried out simply. Further, there is a problem that the gas leakage before or after the detecting process cannot be detected.